Aqueous dispersions of cationic polymers are, for example, used as retention aids in paper manufacturing industry. Other uses are, for example, as flocculants for treating wastewater, as thickeners, and soil improving agents. Generally, these polymer dispersions comprise a dispersed polymer and a dispersant, also called stabiliser, which is usually a polymeric dispersant. The stabiliser stabilises the polymer dispersion in such a way that the dispersed polymer is kept in a dispersed state.
These polymer dispersions are generally prepared by polymerising a reaction mixture of water-soluble monomers in the presence of a first stabiliser acting as a dispersant and a second “co-stabiliser”, which acts as a precipitating agent. Finished polymer will precipitate out and form a stable polymer dispersion. By this no significant amount of inorganic salt is present.
There are a number of criteria that the polymer dispersion should fulfil to give good results in the final application and be of commercial interest. Such criteria are, for example, the process viscosity, active content, stability, good retention and dewatering properties, and easiness of preparing the polymer dispersion including preparing the dispersant. Also, criteria such as environmental and safety aspects are of importance.
By process viscosity is meant the viscosity of the reaction mixture when producing the polymer dispersion. The viscosity should be kept low and viscosity peaks should be avoided, or at least reduced as much as possible, during the production of the polymer dispersion.
There is also desirable that the final viscosity of the polymer dispersion is as low as possible for a certain polymer content for easier handling at use, e.g. pumpability.
The shelf life of the dispersion, i.e., the stability of the polymer dispersion over time, is an important property. An efficient stabiliser is needed for keeping the polymer particles stable in dispersion without settling as sediment.
A further important factor to consider is the active content, i.e., the amount of dispersed polymer in the polymer dispersion. A higher active content gives lower transportation costs and easier handling at the end-application. By using an efficient stabiliser, dispersions with a higher active content can be obtained at the same time the viscosity can be kept low. However, it may be difficult to combine a high active content with good performance in retention and dewatering in a papermaking process.
The cationic charge of a dispersed polymer affects its ability to form stable dispersions. There exist reasons for providing stable polymer dispersions with a comparatively low cationic charge. Such reasons are, for example, FDA limits of cationic monomers for certain use, cost, risk of overcharging the cellulosic suspension when used in papermaking.
During preparation of a polymer-dispersion, deposits of polymer may form and stick to the reaction vessel and stirrer. This leads to time consuming cleaning procedures of the reaction equipment.
WO03/097703 A1 discloses a polymer-dispersion comprising a polymeric stabiliser and also a cationic polyelectrolyte as co-stabiliser. U.S. Pat. No. 6,075,089 discloses a stabiliser which is a graft copolymer.
When preparing polymer dispersions according to above, stabilisers used in prior art processes have been ones prepared in advance. In, for example, WO 03/097703 A1 monomers are polymerised into a dispersed polymer in the presence of a pre-made stabiliser. Thus, the preparation of a stabiliser adds an extra step to the whole polymer dispersion preparation process, requiring time, equipment and leading to costs.
It is an object of the present invention to provide a process for preparing a polymer dispersion which can be made in few steps and in which the process viscosity is kept low and smooth during preparation without any large viscosity peaks, and which gives no deposits. There is a further object of the present invention to provide a polymer dispersion having high stability, comparatively low cationic charge, high active content without too high viscosity, and which at the same time gives good retention and dewatering when used in papermaking processes. There is also an object of the present invention to provide a polymeric stabiliser.